To determine the time course of coronary collateral development, instrumented dogs with coronary occlusions will be studied serially with thallium scanning. Following occlusion, the first scan is to be performed 2 days later with isotope injection during treadmill running. Following injection the animals will be anesthetized to permit scanning over a 4-hour period so that the reperfusion period can be observed. Then scans will be repeated every 5-7 days to measure the reduction in area of ischemia. Using the canine model of LAD occlusion and LCf stenosis, hemodynamics, distal LCf pressure, and myocardial blood flow will be examined following intracoronary Nifedipine injection. By evaluating the effect of the drug on distal LCf pressure and myocardial flow, the presence of steal will be determined.